I thought quotations were for quotes, sarcasm, and unusually emphasized words. I also thought parentheses were for asides. Commas for pauses, or breaks in thought, and colons for making faces. 
I thought italics were never used to indicate sarcasm but instead were used for emphasis. If you were referring to a spineless boss you could sarcastically call him a “fearless leader”. However, put that expression in italics -
my boss is a fearless leader -
and the meaning sure is not what you intended it to be.
I always use quotation marks to mark out an actual “qoute” and inverted commas to show sarcasm for ‘comedy’ value.
Are you saying that that’s the norm?
<sigh> Remember the old days of text only internet BBS and chat? To underline, you’d use an underscore at the beginning and end. So, e.g., the book, To Kill A Mockingbird would look just like that. For italics you’d have to use asterisks for that, just like this. For the emphasis that bolding would provide, we’d use ALL CAPS. Before graphical emoticons, there were the ASCII emoticons, like this --> ;> for a winking evil grin, of course. And even before ASCII emoticons, we used arrow brackets for asides, but you young’uns don’t respect tradition <run, duck, & hide>.
Well, I started out in text only and we emphasized by ALL CAPS or by >andgle brackets<.
Quotes around a term is not necessarily saracasm; it also indicates that the writer thinks what he’s quoting is not the truth: The woman in the senators hotel room was his “wife.”
That’s the main problem with using it to emphasize. By using quotes, you’re deeming the term a lie. In the sentence:
The deputy “testified” in the case.
you’re calling into doubt what he said, not emphasizing it.
Trouble is that if you just use it for emphasis, you get idiot sign writers turning out stuff like this:
COME TO THE BIG JUMBLE SALE
EVERYTHING MUST GO
“FRIDAY 3.30”
SEE U THERE!
I mean, what on earth does that mean? The writer has clearly assumed that the quotation marks signify some kind of emphasis, but has got the implication entirely screwed. In some parts of the UK it is now legal to shoot people on sight if you see them do this. Personally, believing in the might of the pen, I simply carry a large red marker with me and mark stupidly-done signs out of ten.
In other news, am I the only person who was taught at school to call them “inverted commas”?
Where?!?!?! gets his shotgun…
Inverted commas are the symbol - which can be used as an apostrophe, a quotation mark, etc.
italics underlined. Both equally useful in plain-text email.
As to the OP, I recommend Eats Shoots And Leaves. I don’t agree with it all, but it speaks sense. Give your boss a copy.
Sticking quotation marks around a word is more or less equivalent to sticking the phrase “so-called” in front of a word.
Like, if I tell you that my English “teacher” told me to use quotation marks for emphasis, I’d mean, “Well, they call her a teacher, but I’m not saying she actually teaches us.”
This is a fun website with a gallery of misused quotation marks.
Ain’t no such thing as “underlined”. As someone mentioned above, underlining is just a kludge for italic text when it’s not available (as when using a typewriter). Anything underlined should be read as text in italics with low-quality printing. So underscoring text should signify that it’s italic, as when discussing the tittle of a book like Eats Shoots and Leaves or marking text as foreign like tabula rasa.
Asterisking text shouldn’t be thought of as indicating italics, because that would be redundant. The standard I’m familiar with (and the standard Mozilla uses for formatting plain-text mail) uses asterisks to mean bold text.
Using /slashes/ around text is an abomination. It’s hard to read, completely redundant, and it’s a convention already in use for indicating /pro nun see 'ay shun/.
Back to the OP, yes, quotes can reasonably be used to indicate sarcasm.
The boss “testified” at the trial.
Okay, I infer this to mean that during his questioning, he spoke mis-truths. Thus his “testifying” is sarcastic. The quotes act as a stand in for “so-called ‘testified’” in this case. Yeah, that doesn’t read right, but it makes perfect logical sense.
In the same base, italics aren’t appropriate, because you’re not stressing a word even though you’re emphasising it. There’s a lot less sense in reading “the boss testified at the trial.” This stress would indicate to me increduality rather than sarcasm.
And the term for this convention is “scare quotes.”
THERE USED TO BE THIS “SECRETARY” AT WORK. AND ALL HER “MEMOS” WERE IN ALL CAPS. AND SHE USED QUOTES “ALL THE TIME” “FOR NO GOOD REASON” OTHER THAN THAT SHE “FELT” LIKE IT. AND NEARLY EVERYTHING SHE WROTE. HAD TO BE READ “TWICE” AT LEAST TO FIGURE OUT WHAT THE “FUCK” SHE WAS SAYING.
I “really miss” her.
Hmmm
No such thing as underlining
I guess I’ll have to practice my italics next time I write a note in longhand.
GorillaMan made a point back at the start of this thread that I think needs re-emphasising. Using “scare quotes” around testified is incorrect because there is no sense in which the statement that the witness testified is contrary to fact. One does not fail to testify fy committing perjury. One simply testifies falsely.
Furthermore, quotation marks DO NOT indicate sarcasm. Ever. Scare quotes serve to distance the writer from the words used, by implying that they belong to another (even if the other did not actually use those words, or the other is indefinite). This may often be coincident with sarcasm, and is often indicated in speech by a sarcastic tone, but the quotes never indicate sarcasm per se.
My favorite misuse of quotes:
By the way, no offence, ouryL, but I can’t really take anyone in a pedantic language thread seriously who uses “old-fashion” (let alone to describe chat rooms!). I think you loose your pedant’s licence for that. 
After you “loose” it, where does it go? 
ouryL:
I disagree, and I hail from a time when virtually no form of online communication let you do italics or boldface.
If you wanted to underline a word you did this.
If you wanted to boldface or italicize a word you did this.
If you were a blithering idiot you did “this”.